Question about adding Epsom salts to Inground pool

[chem geek]

If you were to add magnesium chloride, that would increase your total conductivity and overall salt level to be too high. You would need to replace some of your water and then add some magnesium chloride essentially substituting sodium chloride for magnesium chloride.

However, I really think this is completely unnecessary and is probably expensive as well. Just keep your calcite saturation index slightly negative -- not higher than -0.2 for example -- and between that and your use of 50 ppm Borates you shouldn't see any flakes from your SWCG and it shouldn't need cleaning very often. Basically if you can avoid formation of calcium carbonate scale, your cell should last longer. Note that you still need to have calcium in your pool because it is concrete, but it is true that with more magnesium in the water as well that the formation of calcium scale will tend to be softer incorporating some magnesium into it, sort of like defects. Magnesium carbonate itself, however, won't directly scale/precipitate since it's solubility product is around 2000 times higher than that of calcium carbonate.

However, note that an SWCG cell will die eventually anyway even if there is no calcium carbonate scale at all. People with low CH pools such as vinyl pools using an SWCG still have to replace their cells as well. They "wear out" due to usage where the electrolysis process slowly damages them mostly by having their special coatings get degraded. The SWCG plates have special coatings (usually with ruthenium oxide as well as iridium over a titanium metal base) that make chlorine production more favorable than oxygen production at the salt levels used in pools. Without such coatings, you'd have to have a much higher chloride level to produce chlorine instead of oxygen gas. The electrolysis process produces heat and microscopic vibration stresses and these slowly wear out the plates both physically as well as forming metal corrosion oxides replacing or plating over the desired coating.

The lifespan is roughly proportional to the current density over time so using an oversized unit and running the SWCG with a lower on-time percentage will have it last longer. Most typical cells are designed with plates that last at least 10,000 hours (some claim 15,000 hours as shown in this link). So if you have an oversized unit where it has an on-time of 25% over an 8-hour circulation pump runtime, then that's 2 hours per day or 14 years lifespan. On the other hand, if you have an on-time of 75% over that 8-hours, you only get 4-1/2 years.